The strong dependence on fossil fuels, more specifically crude oil, in the energy and transport sector where disturbances in the supply have great impact on the economies throughout the world has lead to increased activities in the search for alternative sources for energy.
The increased usage of fossil fuels leads to an increased level of carbon dioxide in the atmosphere, which is strongly believed to have an impact on the global climate. The so-called greenhouse effect is caused by the presence of certain gases such as carbon dioxide, water vapour and methane in the atmosphere. An increased concentration of these gas components in the atmosphere may have an impact of the temperature level and lead to globally warmer climate. Due to the continued and increased usage of fossil fuels the concentration of carbon dioxide increases steadily in the earth's atmosphere with possible severe consequences for economies and basis for life.
The search for reliable and ecologically sustainable solutions to the world's energy demands have been on-going since the first oil crisis in 1974. Conversion of renewable, biomass-based energy sources to electric power and to automotive fuels has however shown to be technically difficult and expensive and only a few real demonstrations have been realized.
The chemical pulping of wood and other lignocellulosic materials is a well-established process to produce pulp and paper products. The most common process is the kraft pulping process where sulphur-and sodium-based chemicals are used when digesting the wood chips into pulp. The invention is also applicable to other chemical pulping processes such as sodium carbonate based non-sulphur processes. At the outlet of the pulp mill's digester step the pulp is separated from the cooking chemicals and dissolved wood constituents of which the lignin is the major part. This separated stream is concentrated in a multi-effect evaporator system to a dryness of 65-85% and is called black liquor. This intermediate stream in a kraft mill is the key energy carrier and provides the major part of the energy required by the kraft process which is a large consumer of electric power and heat.
State-of-the-art technology in a kraft mill for recovery of the energy and the chemicals from the black liquor is to feed it to a recovery boiler, a so-called Tomlinson boiler, where the inorganic cooking chemicals are recovered as a smelt at the bottom of the boiler and withdrawn for recycling to the process and the organic material is combusted and the heat recovered as usable energy by generation of steam.